1. Field of the Invention
The present invention relates to a holding apparatus well suited for use in the transfer or vacuum holding of a thin sheet-type substrate such as a semiconductor substrate (wafer) or a glass plate.
2. Description of the Prior Art
In the manufacturing process of semiconductor integrated circuits, various holding apparatus are used for the purpose of transferring or firmly holding in position a wafer. For instance, in an exposure apparatus performing an important role during the lithographic operation, a substrate vacuum chuck (wafer holder) is used for the purpose of firmly holding a thin sheet-type wafer within a given plane.
Generally, conventional holding apparatus of the above type includes transfer system components or a holder made of a metal or alumina ceramic (sintered Al.sub.2 O.sub.3). Also, Japanese Laid-Open Patent No. 2-186656 proposes the use of a covalent bonding material such as diamond or sapphire for the portions which contact with a wafer so as to prevent the occurrence of fine particles due to abrasive wear.
Then, the holding apparatus of the type used in the manufacturing process of semiconductor integrated circuits, for example, must satisfy the following characteristic properties (a) to (e).
(a) Can prevent the deposition of fine particles (foreign particles) and the occurrence of chemical contamination of an article to be held: If foreign particles deposit on the wafer and the wafer supporting portions of the holding apparatus, this causes not only the breaking of connections and short-circuits in the transferred circuit pattern but also deterioration in the flatness of the wafer and hence deterioration in the resolution due to the deposited fine particles in cases where the holding apparatus is a wafer holder.
(b) Is high in hardness: In order to prevent any abrasive wear at the portions which contact with an article to be held, the contacting portions should preferably be high in hardness.
(c) Is high in rigidity/specific gravity value: In order to hold flat a thin-sheet wafer or to accurately transfer such wafer to a given position, the wafer supporting portions must be high in rigidity. In such case, the wafer supporting portions must be not only high in rigidity but also low in specific gravity in order to make the apparatus small in size and light in weight.
(d) Is low in coefficient of thermal expansion: In the case of the wafer holder of an exposure apparatus, for example, the exposure results in the accummulation of heat. Thus, if the holder is high in coefficient of thermal expansion, this causes an increase in the dimensional variation by exposure of the wafer held by vacuum on the holder. As a result, in order to suppress any dimensional error threatening to occur during the transfer of circuitry, the coefficient of thermal expansion should preferably be as low as possible.
(e) The working is easy: It is desirable that the grindability is so excellent as to produce a smooth surface and that the fine working required for vacuum holding is effected easily.
However, these characteristic properties are not satisfied as yet by the previously mentioned conventional holding apparatus.
To begin with, the holding apparatus of the type using the metal material for the wafer holding portions is not capable of avoiding the metal contamination of a wafer. Also, the metal material is not satisfactory in terms of wear resistance (hardness) and it is also disadvantageous from the standpoint of the rigidity/specific gravity value and the coefficient of thermal expansion.
On the other hand, while the holding apparatus of the type using the alumina ceramic is advantageous in that the rigidity/specific gravity value is high as compared with the metal material thereby ensuring reduction in the size and weight of the apparatus and also the coefficient of thermal expansion is considerably low as compared with the metal material, there are disadvantages that fine particles tend to easily deposit in the large number of pores present in the surface of the sintered alumina ceramic and that the alumina ceramic is an insulator thus causing the deposition of dust due to the generation of static electricity.
Also, with the apparatus proposed in the previously mentioned Japanese Laid-Open Patent No. 2-186656, while it is excellent from the standpoint of wear resistance, in the case of sapphire, for example, the deposition problem of fine particles due to static electricity remains unsolved since sapphire is an insulator. In addition, where the wafer holder, etc., are made of diamond or sapphire, three different methods including a method of using a single crystal material, a method of using a sintered powder and a method of using a surface coating are conceivable and they have the following deficiencies. Firstly, where the single crystal material is used, a large blank material of diamond is not available in the existing situation and a raw material of sapphire is not only expensive but also hard to work it. Further, where the sintered material is used, diamond and sapphire are materials which are hard to sinter. Even if the sintering is possible, it is considered that a great number of large pores are caused and moreover the use of cobalt (Co) or the like as a binder gives rise to a problem of metal contamination. Further, the method of using the coating has the danger of causing the separation of a coating film due to the difference in coefficient of thermal expansion between the coating film and its base material.
As described hereinabove, there has been known in the prior art no holding apparatus which satisfies the required characteristic properties, i.e., the insurance of the desired mechanical strength, the reduction in weight and the dimensional stability and which prevents the contamination of an article to be held such as a wafer due to the deposition of fine particles or the like.